1. Field of the Invention
This invention relates to a developer composition for lithographic printing plates.
2. Description of the Prior Art
Photosensitive polymers of the photocrosslinkable type have been widely used practically as a photo-etching resist. Photosensitive polymers and polymer compositions of this type exhibit excellent characteristics such as sensitivity, stability, image strength, and the like, and are superior to photosensitive diazo materials which have been heretofore used in the preparation of lithographic printing plates. Accordingly, photo-crosslinkable, photosensitive polymers have recently come into widespread use in the preparation of lithographic printing plates. These types of photosensitive polymers crosslink and harden upon exposure to actinic light and become relatively insoluble in solvents in comparison to the unexposed polymer starting material. The differences in properties between exposed areas and unexposed areas of a plate are utilized in the photographic preparation of printing plates. That is, the photosensitive polymer is coated on a lithographic support as a layer together with additives such as, for example, sensitizers, colorants, and the like, and then the plate coated with the layer is exposed to an imagewise pattern of radiation thereby creating soluble and insoluble areas in the photosensitive layer. Thereafter, the plate is treated with a developer solvent or solution for the unexposed polymer to develop the image by removing the unexposed, unhardened, soluble polymer therefrom.
Since it is advantageous to produce an image which is visible at the time of development, a dye or pigment is usually incorporated in the photosensitive layer. In this connection, however, a problem arises in that the proper choice of developer solution is more difficult when these colorants, particularly a pigment, are present. Unless there is a very careful balance in the properties of the developer solution with the properties of the pigment or dye which is used for coloring the photosensitive layer some of the pigment or dye is left to some extent in the unexposed area of the layer upon development which creates stains by deposition of ink on the unexposed area when the plate is used for printing. Accordingly, a developer solution which is used for a lithographic printing plate containing a colored polymer composition must cleanly develop an image on the plate surface without impairing the coated layer in the exposed area by removing unwanted portions of the polymer and by not leaching pigment from the exposed areas.
A further problem with the clean development of lithographic printing plates of the type which have colored coatings of a photosensitive polymer is that plates which have been kept for a considerable time before use are less easily developed than freshly prepared plates, i.e. an unusual amount of staining or scumming takes place on older plates. It is desirable, therefore, that the developer solution produce developed plates without stain and scum not only from fresh plates, but also from aged plates.
In recent years, most of the developments in lithographic plates have been automated and have been achieved by the use of automatic developing machines. In this case, the developer solution is in most cases recirculated repeatedly for reuse. However, when the developer solution which has been employed in such automated processes is used over a period of time, it becomes fatigued and its developing ability diminishes which prevents the lithographic plate from being rapidly and cleanly developed. Therefore, a need continues to exist for a developer solution which exhibits a low rate of fatigue, possesses excellent storage stability and is capable of stable development over a long period of time.
Another aspect of the problem in the lithographic art is that because automatic developing machines are employed which are made of rubber parts, plastic parts, metal parts and the like, contact of the rubber and plastic parts with the developer solution employed may result in swelling and/or dissolution of the rubber or plastic parts, while the metal parts may corrode depending upon the composition of the developer solution. Accordingly, there is a need for developer solutions which exhibit the above-described desirable developing characteristics for lithographic plates and which do not result in damage to the individual parts of the automatic developing machine thereby making it possible to select materials for parts of the developing machine from the widest variety of materials possible.
The developer compositions of the type conventionally used are satisfactory to some extent for cleanly developing lithographic printing plates when they are fresh. However, few standard developer compositions are known which exhibit satisfactory stability characteristics or which do not exhibit a tendency to damage the materials from which automatic developing machines are made. For instance, a standard developer composition which consists of .gamma.-butyrolactone and an acid presents no problems in the clean development of lithographic plates when it is fresh. However, it gives a strongly offensive odor because .gamma.-butyrolactone decomposes during storage or its refining condition is unsatisfactory. Moreover, .gamma.-butyrolactone based developer solutions tend to swell components of automatic developing machines which are made from chloroprene rubbers or silicone rubbers. Therefore, suitable materials from which machine components can be fabricated are limited when .gamma.-butyrolactone based developer solutions are used.
Intensive studies have been conducted on the storage stability of various developer compositions and, as a result, it has been found that the fatigue which the developer solutions of the above-mentioned type exhibits stems greatly from the incorporation of water into the composition, aside from thermal instabilities of the individual components of the developer solution. Even though careful measures are taken to prevent moisture from being incorporated into developer solutions of this type, moisture or water droplets are inevitably absorbed or incorporated into the developer solution when used in automatic developing machines since the developer solution in most cases has to be recycled in practical developing operations and particularly with automatic developing machines. Therefore, a need continues to exist for a developer composition which is usable over a long period of time and which does not incur changes in quality of the developer solution even with the incorporation of given amounts of water in the composition.